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Scintigraphy > Thyroid scan > Flashcards

Thyroid scan Flashcards

1
Q

Thyroid scintigraphy looks are functional or anatomic or both?

A

Both

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2
Q

Hyperthyroid occurs in what aged cats?

A

Middle to old

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3
Q

Cats normally have what type of disease?

A

Benign - adenomatous hyperplasia or hyperfunction adenomas

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4
Q

Thyroid carcinoma is the cause of hyperthyroidism occurs in what % of cats that are hyperthyroid?

A

<3%

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5
Q

What makes relying on T4 difficult in diagnosing hyperthyroid in cats?

A

Day to day fluctuation

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6
Q

Radioactive Iodide is trapped where?

A

salivary glands Gastric mucosa choroid plexus of brain placenta ciliary body of eye lactating mammary gland

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7
Q

Pertechnetate is considered a Pseudohalogen because?

A

it mimics the biologic behavior of iodide and chloride

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8
Q

Peak pertechnetate uptake in the thyroid in dogs? Cats?

A

Dogs: 2-3 hours Cats: 45-60min

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9
Q

When to scan a cat when injected with Tc99?

A

20min - 2 hours

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10
Q

When to scan a dog when injected with Tc99?

A

20min - 2 hours

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11
Q

What is the dose range of Tc99 in a dog? Cat?

A

Dog: 2-5 Cat: 1-4

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12
Q

What are the two mechanisms for thyroglobulin formation or end product of the thyroid?

A

Trapping of iodide Organification

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13
Q

When does trapping happening?

A

<8 hours

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14
Q

When does organification happen?

A

24 hours

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15
Q

Drugs affecting thyroid trapping?

A

Thyroid supplementation - several weeks Sulfonamides - 1-2 weeks Radiographic contrast - 2 months Propanolol ANTI-THYROID MEDS DO NOT AFFECT IT - BIG ADVANTAGE FOR TC99

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16
Q

Drugs affecting organification?

A

Methimazole - This is bad for I123 because this will affect its up take

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17
Q

Which radiopharacuetical produces a picture based on trapping? Organification?

A

Trapping - Tc99 Organification - I123

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18
Q

Cats compare uptake to what salivary gland and what is the normal ratio?

A

Zygomatic 1:1 - 0.6-1

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19
Q

Besides comparing to the salivary glands. what other way of evaluating uptake?

A

Measuring the percentage of injected dose that accumulates in the thyroid - Radio iodine uptake studies (RAIU)

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20
Q

What salivary gland do dogs compare to?

A

Parotid 1:1

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21
Q

RAIU (radioiodine uptake) normals for cat? Dog?

A

0.25-1.5% in Cats in 20 min 0.3-0.5% in Dogs in 20 min

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22
Q

Embryologically where does the thyroid arise?

A

Outpocketing of the endoderm (thyroid diverticulum) from the ventral floor of the pharynx between the first pair of pharyngeal pouches.

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23
Q

Ectopic tissue can be anywhere from the?

A

Base of the tongue to the diaphragm Likes to be next to the aorta.

24
Q

What is the half life of I-123?

A

13.1 hours

25
Q

What is the half life of 99mTc?

A

6 hours

26
Q

What is the ddx for uptake in the mammary glands following injection of 99mTc0- for a thyroid scan?

A

Mammary carcinoma

Mammary hyperplasia

Ectopic thyroid

Metastatic disease

Gynecomastia (pharm induced)

27
Q

Ratio of thyroid to parotid in the dog?

Ratio of thyroid to zygomatic in the cat?

What do these ratios do overtime?

A
  1. 1:1 Dog
  2. 8:1 cat

Overtime (4 hours) the cat ratio will increase and the dog will decrease.

28
Q

Methimazole does what?

A

Stops organification therefore less T4/T3… this will cause increase of TSH from the anterior pituatary gland and therefore more uptake (trapping) of radiopharm in the thyroids.

29
Q

Thyroid:background ratio is used for what in cats? What is the ratio that indicates this?

A

To see which cats need a higher dose of I131

Anything above 13 requires a higher dose.

30
Q

What is the T:S that is confirmatory for hyperthyroid and out of range of normal for a cat?

A

>2.1

31
Q

In Wallacks paper in 2010, what was the most consistent parameter for estimating treatment failure in hyperthyroid cats in scintigraphy?

A

T:B ratio

T:S ratio and T4 levels were insconsistent.

32
Q

Methimazole works how?

A

Reversibly blocks organification of iodid and coupling of iodotyosines. IT DOES NOT BLOCK UPTAKE.

33
Q

What is the percentage of 99mTc uptake in a normal cat?

A

0.64-0.75%

34
Q

When and what ratio is considered the best correlation with serum T4?

A

T:S at 20 min post injection.

35
Q

Fischetti 2005: Says that methimazole treatment (~30 days) did what to quantitative scintigraphy evaluation of the thyroid gland in hyperthyroid cats when compare to a non-treated control?

A

Does not significantly change it.. therefore methimazole treatment will not change the diagnosis in hyperthryoid cats.

36
Q

Fiscetti 2005: Does methimazole treatment (~30days) relieve TSH suppression in hyperthyroid cats?

A

No… methimazole does not relieve TSH suppression by hyperthyroid cats. Meaning TSH does not increase in hyperthyroid cats that are on Methimazole

37
Q

Fiscetti 2005: Methimazole treatment may cause what to happen when you have unilateral hyperthyroid disease in cats?

A

Methimazole will cause the increase in trapping in the normal lobe and therefore make it look hyperthyroid too. The increase in trapping is due to the increase in TSH secondary to the lack of negative feedback from the lack of T4

38
Q

Nieckarz 2001: The T:S ratios were similar at what times post-injection in normal cats and normal cats treated with methimazole? When was it different.

A

20min and 2 hours the T:S ratio was the same

At 4 hours the T:S ratio was significantly increased

39
Q

Nieckarz 2001: Methimazole may cause an increase or decrease in 99mTc trapping and therefore an increase or decrease in uptake in NORMAL cats?

A

Increase in trapping and uptake.

40
Q

What is the rebound effect in methimazole treated cats and why is it important?

A

The rebound effect is a sharp increase in the uptake of radiopharm 24 hours post discontinuation of methimazole. This is clinically important because you can cause significantly more uptake of I131 and therefore cause a hypothyroid cat if treated in this time frame.

41
Q

What pathology in the lungs causes uptake of 99mTc?

A

Primary lung tumors

42
Q

If questionable accumulation of 99mTc in the lungs what is recommended?

A

I123 scan do to more sensitivity and less background noise

43
Q

If a thyroid scan and biopsy confirm malignancy… how long after surgery should you rescan the patient to see if it is all gone?

A

4-6 weeks. Use I123

44
Q

What two forms of hypothyroidism is noted on scinitgraphy in dogs?

A
  1. Thyroid dysgenesis - deminished or no thyroids seen on scintigraphy
  2. Inherited iodinated defects - Large thyroids with normal to increased uptake.
45
Q

What lymph nodes drain the crainal pole of the thyroid vs the caudal pole?

A
  1. Cranial pole is medial retro
  2. Caudal pole is the right lymphatic duct - causing mets to be in the thoracic inlet
46
Q

Venous return from the thryoid is through what?

A

Internal jugular vein.

47
Q

How common is ectopic thyroidal tissue in dogs?

A

Very common - 50% of adult dogs have ectopic thyroidal tissue embedded in the fat on the intrapericardial aorta

48
Q

Why is i123 better for looking for mets than 99mTc?

A

Due to the organification of I123 allowing for later scanning it allows for superior target to background radioactivty.

49
Q

What is another ddx for increased uptake in the lungs after 99mTc scan besides mets?

A

Blood pooling

50
Q

Why should you not do a CT scan before I131?

A

Iodine in the contrast CT will cause the lack of uptake in the thyroid and thus causing your treatment to suck.

51
Q

Differentials for a large photopenic area in a thyroid on 99mTc?

A
  1. Carincoma
  2. Cystic adenoma
52
Q

What releases TRH and what structure releases TSH?

A
  1. TRH = hypothalamus
  2. TSH = Anterior pituitary
53
Q

What is the difference between a radiopharm and a radioisotope?

A
  1. Radioisotope is just 99mTc
  2. Radiopharm is something added to it like MDP
54
Q

What does sedation do to uptake in the thyroid?

A

Increase it…. Acepromazine

55
Q

Embryologically the thyroid in a cat comes from where?

A

endoderm (thyroid diverticulum). Outpocketing from the ventral floor of the pharynx.

56
Q

Is there a difference between the 20 min and 2 hour mark after injection in a cat injected with 99mTc?

A

No

Scintigraphy (8 decks)

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